Flexible micro-electronics circuit packages, such as flexible electronic displays, are often thin sheet-like, band-like, or web-like structures, having a comparatively small thickness or height relative to a length and a width, and formed of one or more substrates, layers, and/or plies. For a flexible electronic display or other flexible electronics article, the ability to be flexed multiple times to a small radius and also the ability to withstand object impact is important for reliability in the field. A dynamically flexible display is a type of flexible display that is adapted to be flexed, bent, shaped, and re-flexed and re-shaped multiple times, and optionally be able to maintain a selected bent shape after being so shaped. For ease of reference, the remaining description will refer primarily to a micro-electronics package in the form of a flexible electronic display; however, the principles of this disclosure are also applicable to other types of micro-electronics packages, such as integrated circuits, solar collectors, etc.
Micro-electronics circuit packages, such as integrated circuit chips and electronic displays, whether flexible or not, are susceptible to developing undesired faults, such as cracks in the circuitry, when put under excessive strains, for example strains caused by bending and/or impacts. One of the main problems is that the circuit elements, such as the electrodes and the switching devices, are often formed of relatively brittle materials that are subject to cracking or buckling when subjected to strains introduced from bending of the substrate on which the circuit elements are disposed. This problem is not overcome simply by making a so-called flexible micro-electronic circuit, such as a flexible electronic display. Rather, the ability of micro-electronics packages to successfully withstand bending without failing often depends on the ability to avoid or withstand excessive amounts of strains within the various electronics layers of the layer stack so as to prevent or minimize any cracking, buckling, forming of gaps, or similar strain-related faults in the electrodes of the various circuits.
A typical flexible electronic display 19, as illustrated in FIG. 1, has three main components: namely, an electro-optic display panel 20, a support substrate 21 disposed on the rear side of the electro-optic display panel 20, for example to support the electro-optic display panel 20, and a front substrate 24, which may include an optional touch sensor 25 and adhesive layer 26, disposed on the front side of the electro-optic display and through which the electro-optic display panel 20 is visible. Typically, the display panel 20 is built and/or processed on the support substrate 21. The flexible electronic display 19 may optionally have a flexible protective substrate including one or more foam layers 22 and adhesive layers 23 disposed on a rear side of the support substrate 21.
FIG. 2 illustrates a detailed cross-section of an exemplary layer stack of the typical flexible electronic display 19 of FIG. 1. The electro-optic display panel 20 includes a layer of electrical circuit components (e.g., a matrix of electrodes, switches, and other circuit components), an electo-optic medium 20a, and an electrode matrix 20b secured to a rear side of the electro-optic medium 20a. Typical electro-optic mediums include liquid crystal displays (LCDs), cholesteric texture liquid crystal (CTLC) displays, Polymer-dispersed liquid crystals (PDLC) display, E-Ink® displays, electrophoretic displays, and electrowetting displays. The electro-optic medium 20a creates and displays changeable visual information, which is visible on the front side of the electro-optic display panel 20, in response to electronic command signals received through the electrode matrix 20b. A front substrate 24 covers and protects the front side of the electro-optic medium 20a. The front substrate 24 and all the layers on top, i.e., everything on top of the electro-optic medium 20a (as depicted in FIG. 2), are often simply called the “top substrate.” In this example, the top substrate includes the front substrate 24 disposed against the front side of the electro-optic medium 20a, a moisture barrier 28 disposed against the front side of the front substrate 24, an optically clear adhesive layer 27 disposed between the front substrate 24 and the moisture barrier 28 to adhere the moisture barrier 28 to the front substrate 24, an adhesive layer 26 disposed against the front side of the moisture barrier 28, and a touch sensor or touch sensitive layer 25 disposed against the front side of the adhesive layer 26. The support substrate 21 (which is also called the “bottom substrate” herein) is typically made of a single layer that carries and/or supports the electrical circuit components of the micro-electronic circuits of the electro-optic display panel 20. The support substrate 21 may be flexible enough to allow a desired flexibility of the overall flexible electronic display 19 while being resilient enough to provide support and/or protection for the electrical circuit components of the electro-optic display panel 20. For ease of reference, the layers from the bottom substrate 21 to the top-most layer, in this case the touch sensor 25, are also referred to collectively as the display stack 29.
The foam layer 22 and the adhesive layer 23, which form a backing structure stack 30, may be attached to the rear side of the display stack 29 on the bottom substrate 21 to protect the display stack from impacts. As illustrated in FIG. 2, the adhesive layer 23 is typically disposed against the rear side of the support substrate 21, and the foam layer 22 is disposed against the rear side of the adhesive layer 23. The foam layer 22 is a relatively thick layer of foam having visco-elastic properties and a stiffness dependent on the speed of compression, in that the stiffness is very low when flexed or impacted slowly and the stiffness increases when the foam is indented or impacted at a high rate of speed. This feature results in the foam layer 22 not affecting the overall flexibility of the display 19 to any significant respect, but at the same time protects the display from impact by effectively spreading the forces from an impact over a larger area.
Before proceeding further, it is noted here that the terms “dynamic bending,” “dynamic flexibility,” “dynamically flexible,” and similar terms, for example, as used herein generally refer to the ability to bend the attachable article, and more particularly the flexible electronic component, at a number of different points, if not every point, along a length of the flexible component, the ability to bend different portions of the attachable article differently (e.g., different portions can be bent at/to different angles or curvatures), the ability to bend the attachable article in a number of different directions (e.g., in a concave direction and a convex direction), and/or the ability to bend the attachable article in some other dynamic manner. Further, dynamically flexible may refer to the ability to repeatedly bend (i.e., multiple times) in different manners and directions, for example, so to as be selectively formable and re-formable to various different shapes by a user, without damaging the intended operable usability of the article, such as would be usual and customary when used as a wrist band, belt, item of clothing, and the like. In addition, as used herein, the “front” side of the display panel 20 is the side that produces changeable optical images intended to be seen by a viewer, and the “rear” or “back” side of the display panel is the opposite side, which typically is not designed to display any particular images to be seen by the user.
Following is a short discussion about basic mechanical properties of a typical flexible electronic display. Because flexible electronic displays are typically produced on a flat surface, a curvature creates a certain strain profile in the display. Also, there can already be certain strains and stresses in the electronic display without bending due to the processing conditions of the display, such as temperature induced stress. If the electronic display is flexed too much, such that the curvature is too high, the strains developed in the electronic display can cause certain brittle layers in the display stack 29 to buckle or crack, which of course may cause the electronic display to malfunction. Typically the brittle, inorganic layers in a display, such as SiNx and/or ITO, can withstand roughly a 1% strain without buckling or cracking, depending on the amount of built-in stress and other process conditions, such as layer thickness, associated with the display. Organic layers, such as planarization layers and plastics, can typically withstand strains up to 8% without breaking or deforming in a non-elastic way.
As is well known from basic material mechanics, when a beam of any type—and for purposes of this discussion, the electronic display—is flexed, the outer radius of the flexed region is under tension, while the inner radius of the flexed region is under compression. Somewhere between the outer radius and the inner radius of the layer stack is the neutral plane (or neutral axis in a 2-D representation), where there is no tension or compression upon bending. Therefore, in a typical electronic display, such as the display stack 29, the brittle layers of the display stack that are furthest away from the neutral axis will buckle or crack first, i.e., before the layers that are close to the neutral axis, when the electronic display is bent beyond its limits, i.e., with a radius of curvature that is too short.
The position of the neutral axis in the electronic display may be approximated with the following formula:Yn=(Σ(Ti*Ei*Yi))/(Σ(Ti*Ei)),  Eq. (1)
where Yn is a distance normally extending from a reference plane to the neutral line, Ti is the thickness of a respective layer of the flexible display assembly, Ei is an elastic modulus of a material placed in the respective layer of the flexible display assembly, Yi is a distance normally extending from the reference plane to the geometric center of the respective layer. Although this equation provides a simple first order approximation that does not take into account local patterning of the layers, creep in the layers, or compensating effects by deforming (visco) elastic layers, this equation provides a reasonable approximation for purposes of this disclosure. An example calculation of the neutral axis using equation 1, where the point Yi=0 has been defined as the back side of the bottom substrate 21 in FIG. 1, is provided in Table 1 below:
TABLE 1Young's Ei × modulusThickness Ei × Position Ti × LayerEi (GPa)Ti (μm)TiYi (μm)Yi1. Substrate5251251315632. Brittle layer1000.110252513. Driving electronics510503015054. Electro-optical layer 0.1505603015. Brittle layer1000.110858526. Substrate5251259812213Sum32516683Yn (μm)51
When the display (or a part of the display) is bent over a radius ρ, the strain ε at a given location in the thickness of the display is equal to:ε=ΔL/L=(L(A′B′)−L(AB))/L(AB)=y/ρ,  Eq. (2)
where y is the distance from the neutral plane, being negative for the compressed part of the display, resulting in a negative strain for compression and a positive strain for tension.
If, for example, the brittle layers of the schematic display of FIG. 2 would break beyond a tensile or compressive strain of 1%, the following would be the relation between the minimum bending radius and the distance from the neutral axis:ρmin=y/1%  Eq. (3)
Using the numbers developed in Table 1 results in a minimum bending radius of 2.6 mm for the first brittle layer and 3.4 mm for the second brittle layer.
In general, there are two primary scenarios where the strain limits of the layers of the electro-optic display panel 20 can be reached due to localized bending from the impact of an object such that undesirable cracking, buckling, and/or formation of gaps in the electrodes may occur. A first scenario occurs during impact of an object at a relatively low velocity, such as when the layers of the display are pressed slowly and steadily by a finger or pencil or other item. One common low-velocity impact situation encountered during normal daily use typically includes an elbow that bumps into the device, for example with a radius of about 10 mm, a mass of about 500 g, and an impact velocity of about 0.3 m/s. Another common low-velocity impact situation encountered during normal daily use typically includes a stylus pushing into the display, for example with a radius of about 1 mm, a mass of about 100 g, and an impact velocity of about 0.1 m/s. Yet another common low-velocity impact situation encountered during normal daily use typically includes a stylus or finger nail moving over the display while pushing, for example, with a radius of about 1 mm, a mass of about 100 g, and a lateral velocity of about 0.2 m/s. In this later example, there is typically a lateral movement that can be translated into a new display surface area being pressed by the stylus or the finger nail, where the impact velocity is similar to the lateral velocity. In general, a low velocity impact as used herein has an impact with a velocity typically lower than about 1 m/s, more typically a velocity lower than about 0.5 m/s, and that can have a velocity as low as about 0.01 m/s. Low-velocity impacts generally cause a local pressure on the electronic circuits that is applied relatively slowly and with a relatively long duration. The duration of an impact as used here is the time it takes to reach peak indentation. That is, for example, the elapsed time from when an object first touches the surface of the display panel until the time the object most deeply indents the display panel. This duration is typically short, such as less than one second. In contrast, the duration of an indentation in cases of the normal daily uses above may sometimes be longer, but that would be keeping the indentation at a certain indentation level, for example, by keeping the stylus at the same spot for a while or moving the stylus the display. The duration of an indentation can be as long as multiple seconds.
A second scenario, shown schematically in FIG. 3, occurs during a relatively high-velocity impact on the electronic display, during which the object impacting the display creates a localized spot where the individual layers of the flexible electrical display are stretched in only a small region of the overall surface area of the display. FIG. 3 illustrates a schematic cross section of the display stack when impacted at a high rate of speed by a spherical ball 31, for example. When the electro-optic medium 20a of the display, such as a liquid crystal, is either a fluid or a polymer with fluidic pockets, capsules or a polymer network in a fluid (LCD, CTLC, PDLC, E Ink, electrophoretics, electrowetting), this medium will have a Young's modulus that is orders of magnitude lower than that of either the plastic or glass present in the other layers of the display stack 29. Therefore, in general, this medium also has a much lower flexural rigidity than either plastic or glass. An impact event that typically causes a high-velocity impact is, for example, a solar cell device located outdoors that requires a resistance to extreme hail conditions, wherein the device can preferably withstand an impact equivalent to a ball with a radius of about 30 cm, a mass of about 104 g, and an impact velocity of about 15 m/s. Another impact event that typically causes a high-velocity impact is in rough environments and may be modeled as, for example, a steel ball with a radius of about 25 mm, a mass of about 510 g, and an impact velocity of about 5 m/s. Another impact event that typically causes a high-velocity impact in less rough environments may be modeled as, for example, a billiard ball having a radius of about 29.5 mm, a mass of about 139 g, and an impact velocity of about 5 m/s, or for example a steel ball having a radius of about 9.5 mm, a mass of about 28 g, and an impact velocity of about 2.5 m/s.
For a flexible electronic display or another flexible micro-electronics article including a layer stack as outlined above, the preferred materials from which the circuits are made (e.g. metals like Au, Ag, Cu, ITO, Mo, Al) are often brittle, i.e., have low ductility, and therefore are subject to cracking, buckling, and breaking under strains developed, for example, when the electronics circuit undergoes bending caused by either low-velocity impacts or by high-velocity impacts as described above. Such cracks can form gaps along the circuit electrodes, for example on the order of 1 nanometer to 500 nanometers and even up to which can degrade or even prevent conduction of electrical current through the electrodes, thereby degrading or preventing proper functioning of the circuit.
Different ways to protect the electrical circuit components from developing faults due to bending and impacts have been attempted. For example, in one manner, as illustrated generally by WO 2008/133513 A1, an elastic material is adhered to the back of the display to absorb the impact energy while the display does not experience a pressure or stretching that is too high. In terms of the example shown in FIG. 2, the layer 22 of the protective backing stack 30 would be formed of an elastic material, such as rubber. Although this arrangement may be helpful for protecting the electrical circuit components against low velocity impacts, it is not as helpful for protecting against high-velocity impacts.
In another manner, as illustrated generally by EP 2551110 B1, a visco-elastic material is adhered to the back of the display in order to absorb high velocity impact. The material is selected such that it does not affect the position of the neutral plane and the minimum bending radius of the display significantly. In terms of the example shown in FIG. 2, the layer 22 of the protective backing stack 30 would be formed of a highly viscoelastic material, such as a foam. Although this arrangement may be helpful for protecting the electrical circuit components against high-velocity impacts, it is not as helpful for protecting against low-velocity impacts.
However, the problem of optimizing a protective backing material to protect the electronic circuit from both low-velocity impacts and high-velocity impacts has not yet been addressed in a satisfactory manner.